The present invention relates to electric meters and, more specifically, to registers for electric meters.
When an electric utility operates near the upper limit of its power-generating capacity, it is valuable to find a way to limit the total power consumption of its customers during peak hours in order to avoid overloading its generator system. Utilities attempt to accomplish such limiting by applying a higher tariff for high level of consumption during peak times.
Many rate-setting bodies permit the electric utility to use multitariff systems in which a higher tariff is applied to the energy consumed at an excessive rate, or at certain times, or at combinations of these parameters. In order to implement a multitariff system, an electric meter must include means for recording power usage at a higher rate under the defined conditions. A current threshold above which a higher tariff applies in a 220-volt single-phase system may be selected from, for example, 15, 18, 24, 30, 36, and 45 amperes according to the consumer's prior usage. Below the threshold, the normal tariff is applied. Above about 45 amperes (equivalent to 14 KVA in a 220 volt, 50 Hertz, network), a three-phase system may be preferred.
In the prior art, such differential tariffs are accomplished with demand registers in which the energy usage is totalled over each of a series of fixed demand intervals. A typical demand register is disclosed in U.S. Pat. No. 3,586,974. According to the referenced patent, if the energy usage in any one demand interval exceeds a threshold value over a period between meter readings, the higher of two tariffs is applied. Although effective in limiting peaks in energy usage, a demand register, integrating energy usage over fixed demand intervals, tends to average out usage peaks and weakens the incentive for limiting instantaneous demand. In addition, conventional demand registers are not equipped to apply the higher tariff, or ignore it, depending on the time of day.
In order to implement some multitariff systems, a utility requires an electric meter of a type including means for separating and separately accumulating the energy usage according to the instantaneous power level at which such usage occurs and/or according to the time of day. The threshold should preferably be relatively simply changed by the utility and should be relatively difficult for the consumer to modify.